Thx a lot for your answer.

I finally managed this way:

CREATE MEMBER CURRENTCUBE.[Measures].[Full Year]

AS AGGREGATE(

ANCESTOR( [Time].[Year - Month].CurrentMember,[Time].[Year - Month].[Year] ), [Measures].[Value]),

FORMAT_STRING = "#,##0.00;-#,##0.00",

VISIBLE = 1 , DISPLAY_FOLDER = 'Calculated measures' , ASSOCIATED_MEASURE_GROUP = 'Line Items' ;